Usually, the presence of biologically active agents in a patient's body fluid, and especially in blood, is determined using blood culture vials. A small quantity of blood is typically injected through an enclosing rubber septum into a sterile vial containing a culture medium. The vial is typically incubated at 37.degree. C. and monitored for bacterial growth.
Common visual inspection for bacterial growth involves monitoring the turbidity of the liquid suspension. Known instrumented methods detect changes in the carbon dioxide content of the culture bottles, which is a metabolic by-product of the bacterial growth. Monitoring the carbon dioxide content can be accomplished by methods well-established in the art, such as radiochemical (e.g., BACTEC.RTM., Becton-Dickinson, Franklin Lakes, N.J., U.S.A.), infrared absorption at a carbon dioxide spectral line (e.g., NR-BACTEC.RTM., Becton-Dickinson, Franklin Lakes, N.J., U.S.A.), or pressure/vacuum measurement techniques such as those disclosed in U.S. Pat. No. 4,152,213--Ahnell. These methods, however, all require invasive procedures which result in the well-known problem of cross-contamination. As used herein, the term invasive describes a procedure wherein the sample container is opened, pierced or otherwise placed in communication with an external environment during the point at which the presence or absence of bacteria is determined.
Recently, non-invasive methods have been developed involving chemical sensors disposed inside the sample vial. These sensors respond to changes in carbon dioxide concentration by changing color or by changing fluorescence intensity. These techniques are based on light intensity measurements, thus errors occur if the light sources used to excite the sensors or the photodetectors used to monitor intensities show aging effects over time. Certain of the disadvantages of such intensity-based methods could be overcome by utilizing modulated excitation light in combination with fluorescent sensors that change their decay time with changing carbon dioxide concentration. In this case, intensity measurement is replaced with time measurement, and intensity changes have no impact. However, current fluorescent decay time sensors require high-brightness, short-wavelength light sources (550 nm or shorter) that are intensity-modulated at very high frequencies (typically above 100 MHz). An example of such a device would be a 5-mW green HeNe laser (543.5 nm) externally modulated by means of an acousto-optic light modulator. However, as well known by those of ordinary skill, the laser/modulator combination is rather expensive, requiring that the individual samples be moved to the laser instead of having one light source at each sample. Such an instrument would necessarily have a complicated mechanism for effecting the transportation of the individual samples, and the time interval between successive measurements for each sample would be relatively long. Since for the time being it appears unlikely that high-brightness, short-wavelength semiconductor diode lasers will be developed to permit a commercially feasible embodiment of this type of system, such an improved system would suffer serious practical shortcomings.
A need therefore remains to provide methods and apparatus for bacteria detection in blood culture samples non-invasively and in a commercially feasible manner. It is therefore an object of the present invention to provide methods and apparatus for detecting bacteria in a sample that permit a plurality of samples to be tested simultaneously in a rapid, effective and economical manner.